This invention relates generally to the purification of semiconductor etching solutions and more particularly to a method of removing metal ions from ammonium fluoride solutions.
Ammonium fluoride solutions, normally mixed with hydrofluoric acid are used in great quantities for semiconductor etching operations. Such solutions contain traces of impurities of, for example, copper, arsenic, gold, palladium, and silver which because of their positive electrode potential are irreversibly deposited on the semiconductor surfaces during the manufacturing process, causing serious electrical defects, such as inversions and gate shorts in field-effect transistors. Particularly detrimental is arsenic which as a result of its valence V leads to surface inversions, in particular in the layers that are slightly p doped, and thus to frequent failures when used in conjunction with insulated gate field-effect transistors that are being employed to an ever increasing extent. Therefore, it is essential to use ammonium fluoride solutions in which the impurity traces mentioned do not exceed quantities of .ltoreq. 2 .times. 10.sup..sup.-6 percent by weight. Certain applications require purities of even less than 10.sup..sup.-7 percent by weight of arsenic. However, commercially available ammonium fluoride solutions contain as much as 10.sup..sup.-5 percent by weight of arsenic. From this if follows that methods of generating ammonium fluoride solutions with the required percentage purity are of great importance. The following description is essentially related to arsenic. This element is particularly detrimental, being more difficult to remove than other impurity traces, and the methods used to remove it are at least equally suitable for the removal of other inpurity traces. In view of this, the following description applies in analogy to other impurity traces.
Generally ammonium fluoride is produced from ammonia and hydrofluoric acid, and most known processes for producing highly pure ammonium fluoride solutions are based on processes permitting the basic materials to be produced without difficulty, but it is more problematical to produce hydrofluoric acid with an arsenic content of less than 2 .times. 10.sup..sup.-6 percent by weight when the fluorspar (CaF.sub.2) from which the hydrofluoric acid is formed contains great quantities of arsenic. There are, it is true, fluorspar deposits whose arsenic content is so low that the hydrofluoric acid formed from them meets the purity standards without additional purification steps. But fluorspar of such purity is not always available. German Pat. No. 1,290,529 describes a method of removing arsenic impurities from hydrofluoric acid, whereby the impurities are separated by distilling the hydrofluoric acid, which in the case of arsenic necessitates that the arsenic be initially converted into a high-boiling valence V compound by adding oxidizing agents, such as potassium permanganate or iodine. A similar method is described in German Patent No. 1,767,548. As is shown in the drawing accompanying Pat. No. 1,767,548, a highly complicated apparatus is necessary for executing the above-mentioned purification method. In addition, there are material and safety problems which occur in particular when working with hot hydrofluoric acid. Another method of purifying hydrofluoric acid which requires a less elaborate apparatus is described in German Pat. No. 1,221,614. The method in accordance with Pat. No. 1,221,614 utilizes the property of arsenic of irreversibly depositing itself on semiconductor surfaces. During the distillation process of hydrofluoric acid, the acid vapor is passed through a column packed with a higher pure semiconductor material. Such packings are, of course, very expensive, so that the latter method is used only for special studies on a laboratory scale requiring small quantities of highly pure hydrofluoric acid. Apart from the difficulties described, these known methods have the disadvantage of being applicable only to hydrofluoric acid alone but not to ammonium fluoride solutions which cannot be distilled because of their high salt content. Therefore, previously known methods are unsuitable for removing impurities from ammonium fluoride solutions which, for example, have entered the solutions as a result of prolonged storage and for which subsequent purification steps are necessary if the analyzed arsenic content is too high or where used ammonium fluoride solutions have to be regenerated.
Therefore, it is the object of the invention to provide a method permitting impurity traces to be rapidly and reproducibly removed directly from ammonium fluoride solutions down to a residual content of less then 2 .times. 10.sup..sup.-6 percent by weight or, if possible, down to a value which is 1 to 2 orders lower than the latter value. It is a further object that this method must be suitable for use under manufacturing conditions, taking into account the special safety risk involved, and that the equipment required is not too elaborate.